This invention relates to a new photovoltaic cell of the multijunction type which is based upon the quantum well effect.
Solar radiation is received as a broad spectrum spanning predominantly the UV through infrared ranges. No single semiconductor junction can efficiently convert this entire spectrum to electricity. Hence, many different multijunction structures grown or stacked on top of one another to form composite solar cells have been proposed in the past. See, e.g., Bedair et al, IEEE Transactions on Electron Devices, Vol.ED-27, No.4, Apr. 1980, page 822; and U.S. Pat. Nos. 3,186,873; 3,478,214; 4,017,332; 4,099,199; 4,128,733; 4,179,702; 4,206,002; and 4,255,211.
One problem encountered in such multijunction devices is derived from the fact that the semiconductor materials in adjacent layers are normally not lattice matched. Furthermore, as Bedair et al, supra, have shown, there are no non-alloy materials which have optimum energy gaps for use in solar cells and which, at the same time, are lattice matched. As a result, in most multijunction solar cells, crystal defects are introduced which cause performance degradation.
One proposed solution is to grow layers of graded composition onto a substrate, e.g., layers of compounds or elements of Group III-V of the Periodic Table on a semiconductor substrate, e.g., GaP.sub.x As.sub.1-x on silicon or Ge.sub.1-x Si.sub.x on silicon, starting with x=1 at the substrate and gradually increasing the value of x as the layer is grown thicker. At the top end, the lattice constant of the layer of changing composition more nearly matches that of the layer which will be placed thereupon. See, e.g., Fan et al, Appl. Phys. Lett. 37 (11), Dec. 1 1980, page 1024, or U.S. Pat. Nos. 4,206,002 or 4,128,733. However, this process is quite difficult to implement and very often still results in crystalline defects which seriously degrade performance.
Another proposed solution is to grow a strained-layer superlattice between lattice mismatched layers. See, e.g., Matthews et al, J. Vac. Sci. Technol., Vol. 14, No. 4, July/August 1977, page 989. The superlattice consists of very thin alternating layers of dissimilar semiconductor materials. By appropriate selection of thicknesses, the combination of thin layers greatly minimizes the propagation of dislocations from the underlying layers to the upper layers.
It is also known that changes in energy band structure occur when thin layers of dissimilar semiconductor materials are grown on top of each other. These thin layers have energy levels which are different from the bulk semiconductor material from which they are grown. This is a quantum mechanical effect occurring as the layer thicknesses approach the crystal lattice constant. The general, repeating structure is that of a thin semiconductor layer, which in bulk form would have a given bandgap energy, disposed between two layers of another semiconductor material having a wider bandgap energy in bulk. As a result, the bandgap energy of the middle sandwiched layers is between those of the two "bulk" materials as long as the middle layers are sufficiently thin that the quantum mechanical effect occurs. These thin layers thus constitute "quantum wells" which have bandgaps lower than those of the surrounding layers. See, e.g., Panisch, Science, Vol. 208, May 23, 1980, 916-922; L. V. Keldysh, Sov. Phys. Solid State 4, 1658 (1963); L. Esaki and R. Tsu, IBM J. Res. Dev. 14, 61 (1970); R. Dingle et al, Phys. Rev. Lett. 33, 827 (1974); L. L. Chang et al, Appl. Phys. Lett. 28, 39 (1976); R. M. Fleming et al, J. Appl. Phys. 51, 357 (1980); R. Dingle et al, Appl. Phys. Lett. 33, 665 (1978); W. T. Tsang et al, ibid. 35, 673 (1979).